1. Field of the Invention
The present invention relates to a power driven tool and particularly to a power driven tool for crimping a collar which is fitted on a shank of a rivet for fastening a work.
2. Description of the Prior Art
A conventional power driven tool of the above-mentioned type includes an inner sleeve having a chuck for grasping a shank of a rivet, an outer sleeve coaxially disposed with the inner sleeve and having a crimping member for crimping a collar, and a motor for driving the inner sleeve. In operation, the shank of the rivet is inserted into a work to be fastened and thereafter a collar is fitted on the shank of the rivet. The end portion of the shank is grasped by the chuck of the inner sleeve and the inner sleeve is moved in an axial direction to pull the rivet. Then, the crimping member is forced to be fitted on the collar, so that the collar is crimped and fastened on the shank, and thereafter, a part of the shank of the rivet is cut by the crimping member.
The inner sleeve is reciprocally moved by the motor in such a manner that it moves rearwardly to pull the shank of the rivet while it moves forwardly to restore its position for engagement of the chuck with the shank. Various mechanisms have been proposed heretofore to convert the rotation of the motor into liner reciprocal movement.
Japanese Patent Publication No. 49-14033 discloses a power driven tool having a hydraulic piston-cylinder mechanism for moving an inner sleeve. This mechanism, however, requires a relatively long fluid pipe, a gear pump and various valve means, etc. The power driven tool having this mechanism tends to become large and weighty, and therefore, it cannot be effectively operated.
U.S. Pat. No. 3,029,665 discloses a power driven tool having a fork which is connected with an inner sleeve and is swung around a pivot. With this mechanism, the fork has to be relatively long, and therefore, the power driven tool tends to become large.
Japanese Utility Model Publication No. 55-37335 discloses a power driven tool having a ball screw mechanism for moving an inner sleeve. The ball screw mechanism includes a threaded shaft driven by a motor through harmonic gear means and a threaded portion formed on the inner surface of the inner sleeve for engagement with the threaded shaft through a plurality of balls. With such construction, the inner sleeve is moved forwardly when the motor is rotated in one direction, while it is moved rearwardly when the motor is rotated in another direction. An operational rod and a proximity switch are provided to change the change the direction of rotation of the motor. The operational rod extends outwardly from a gear case accommodating the harmonic gear means and moves together with the inner sleeve. The proximity switch detects the end of the operational rod and outputs a control signal of the motor for firstly stopping the rotation of the motor and for thereafter starting the motor for rotation in another direction. The ball screw mechanism is relatively compact and lightweight as compared with the above-mentioned hydraulic piston-cylinder mechanism or the swinging fork mechanism. However, since the direction of rotation of the motor is changed by the operational rod and the proximity switch, the motor may further rotate by an inertia force even after it has been stopped by the control signal. This may consequently cause excessive movement of the inner sleeve in the forward or rearward direction from the stroke ends defined by the position of the proximity switch. Therefore, it requires to determine the position of the proximity switch in consideration of the excessive movement of the inner sleeve after the motor has been stopped. Such positioning of the proximity switch cannot be reliably made and therefore, the stroke ends of the inner sleeve cannot be accurately positioned. Additionally, since the operational rod extends in an axial direction and moves together with the inner sleeve, the whole tool requires to be relatively long, so that the operation of the power driven tool cannot be reliably performed.